1. Field of the Invention
The present invention relates to a light emitting diode and a method of fabricating the same, and more particularly, to a light emitting diode with improved electrostatic discharge characteristics and/or luminous efficiency and a method of fabricating the same.
2. Discussion of the Background
Generally, a gallium nitride (GaN)-based semiconductor may be used for an ultraviolet or blue/green light emitting diode or laser diode, or the like, as a light source for a full-color display, a traffic signal lamp, a general lighting, and optical communication devices. The GaN-based light emitting device may include an active layer having an indium gallium nitride (InGaN)-based multi-quantum well structure disposed between n-type and p-type GaN semiconductor layers, and may generate and emit light by recombination of electrons and holes in the quantum well layer in the active layer.
FIG. 1 is a cross-sectional view of a light emitting diode according to related art.
Referring to FIG. 1, the light emitting diode includes a substrate 11, a low-temperature buffer layer or nucleation layer 13, an undoped GaN layer 15, an n-type contact layer 17, an active region 25, and a p type contact layer 27.
The light emitting diode according to the related art includes the active region 25 having the multi-quantum well structure disposed between the n-type contact layer 17 and the p-type contact layer 27, which may improve luminous efficiency. Further, the light emitting diode controls indium content of an InGaN well layer within the multi-quantum well structure, which may allow light emission of a desired wavelength.
The n-type contact layer 17 generally may have a doping concentration (i.e. number density) ranging from 1018 cm−3 to 1019 cm−3 and may serve to supply electrons in the light emitting diode. The current spreading performance within the light emitting diode may have a large effect on the luminous efficiency of the light emitting diode. When the n-type contact layer 17 and the p-type contact layer 27 are respectively provided with an n-electrode and a p-electrode (not shown), current concentration may occur according to a size of an area and a position in which the n-electrode and p-electrode contact the contact layers 17 and 27. When high voltage such as electrostatic discharge (ESD) is applied to the light emitting diode, ESD breakdown of the light emitting diode may easily occur due to the current concentration. In addition, thread dislocations may be generated from the low-temperature buffer layer 13 and may be transferred to the undoped GaN layer 15, the n-type contact layer 17, the active region 25, and the p-type contact layer 27. Since current may flow intensively through these thread dislocations, the ESD characteristics may lead to further deterioration of the light emitting diode.
In addition, since there may be about 11% of lattice mis-match between GaN and InN, an interfacial strain may occur between a quantum well layer and a barrier layer in the InGaN-based multi-quantum well structure. This strain may cause a piezoelectric field in the quantum well layer, thereby leading to degradation of internal quantum efficiency. In particular, in the case of a green light emitting diode, since the amount of In contained in the quantum well may be greater compared to other wavelengths, the internal quantum efficiency may be further reduced by the piezoelectric field.
In the InGaN light emitting diode, the active region having the multi-quantum well structure may generally be formed by alternately stacking the InGaN well layer and the InGaN barrier layer. The well layer is formed of a semiconductor layer having a smaller bandgap than that of the barrier layer and electrons and holes are recombined in the well layer. In addition, the barrier layers may be doped with silicon (Si) in order to lower a forward voltage Vf. However, the Si doping may have a negative effect on the crystal quality of the active region. Further, due to the limitations of epitaxial growth technology, the multi-quantum well structure may be relatively thick according to the doping of Si. In particular, when Si is doped in the active region including In, crystal defects may frequently occur on the surface of the active region and in the active region and a wavelength shift may be easily generated due to a space charge separation generated by a polarization field.
Meanwhile, the external quantum efficiency of the light emitting diode may increase with an increase in the injection current under low current conditions, while the external quantum efficiency may be degraded with an increase in the injection current under high current conditions. This phenomenon is referred to as an efficiency droop, which may limit the efficiency of a high-out put light emitting diode.
Factors that may cause efficiency droop are thermal vibration. Auger recombination, internal field within the multi-quantum well structure, non-recombination rate due to the crystal structure, etc.
Electrons and holes may not stay long in the active layer region due to the thermal vibration according to thermal or Joule heating, thereby making it possible to cause the efficiency droop. The efficiency droop may be caused by the occurrence of the Auger recombination due to the increase in carrier concentration when high current is injected. Further, the efficiency droop may be caused with the increase in non-recombination rate due to electron overflow daring application of high voltage, and the efficiency droop may be caused with increase of the non-radiative recombination rate due to the defects in a semiconductor crystal.
Meanwhile, an AlGaN electron blocking layer (EBL) may be formed on the active layer in order to prevent electrons from flowing out the active layer. However, an internal field may be generated by spontaneous polarization and piezo polarization in the active layer and the electron blocking layer. Due to the internal field within the active layer and the electron blocking layer, high voltage should be applied in order to pass electrons through the active layer having the multi-quantum well structure. In particular, if the applied voltage is larger than a built-in voltage in a 350 mA high-output diode, a conduction band at an n-type side may have a higher energy level than a conduction band at a p-type side, based on the center of the active layer. The energy level of the electron blocking layer may be lowered, which may increase leakage current. In order to increase the energy level of the electron blocking layer, an aluminum (Al) composition may be added to or be increased within the electron blocking layer; however, the increase may degrade crystal quality of the light emitting diode.